Fox 35 Problem

I raced my Fox .35 stunt last year and it was running great. Then we had a contest where the outside air temp was about 120 degrees.

On the pit stops the venturi would get primed as the tank got full but, you had to get the battery on and start it within 2 seconds because, the prime was boiling.

Engines were catching fire, the back of plastic props were melting. Hottest day I've ever flown a plane.

The competition was close so I had to keep it leaned down for the win but, I think it did something.

Anyway, the piston feels sticky now from exhast closing through TDC when I pull it through. It still starts and runs ok but, is this just varnish or did the piston get mashed out. Do you think a couple of Luxdrops would fix it?

I would bet that it's lots of varnish.

Run a couple of tankfuls of all synthetic fuel through the engine and see if it improves. This supposedly removes varnish.

I tend agree with Walter. If so, you should be able to see the brown crud on the side of the piston. Try running a short tank of something with about 5% synthetic oil. A bit too little is way better than a bit too much, as you can repeat as necessary. To a degree, the Fox depends on some varnish for the piston seal, and too much synthetic oil will remove too much of the varnish. Good luck.

Thanks for the replies.

I found the problem, the piston is cracked from crown to skirt and the wrist pin boss is broken out the top on one side.

Funny thing is I had it running in a race today. It ran poorly and took 15 minutes to finish but, never went ca-chunk.

I've got a good used piston/sleeve I'm going to try in it after I scrape all the carbon off the head.

Remember, for all the bad things people want to say about a Fox, if you return it to the factory, they will repair it for just the cost of parts. If the cost of parts exceeds 50% of a new (list price) engine, they will send you a new one for that same 50% off.

I have broken three cranks over the last 15 years, and sent the engines to Fox. They replaced the cranks in all three no charge. All of these Fox 35's had lots of hours of run time on them. Each time I sent one in, I had it back in a couple of days no charge. They didn't even charge me for the cost to mail them back to me. You just cannot beat service like that.